1. Field of the Invention
The present invention generally relates to wireless communications. More particularly, the invention relates to a channelization scheme for a wireless network. Still more particularly, the invention relates to a four channel channelization scheme for an IEEE 802.11 wireless network operating in the Industrial, Scientific, Medical frequency band between 2400 and 2483.5 MHz.
2. Background Information
Initially, computers were most typically used in a standalone manner. It is now commonplace for computers and other types of electronic devices to communicate with each other. The ability for computers to communicate with one another has lead to the creation of small networks comprising two or three computers to vast networks comprising hundreds or even thousands of computers. Networks thus can be set up to provide a wide assortment of capabilities. For example, networked computers can be established so as to permit each computer to share a centralized mass storage device or printer. Further, networks enable electronic mail and numerous other types of services. Networks can be established in a wired configuration in which each entity on the network has a direct physical electrical connection to the network. More recently, wireless technology has increased in popularity to provide wireless data communications to the entities on the network.
Wireless networks are defined in various standards. The IEEE 802.11 standard, for example, permits a wireless local area network (WLAN) to be set up in a business or residence. One or more “access points” are located at predetermined locations and are connected via cables to servers and other types of network equipment. Each access point also has a wireless radio capability to permit wireless communications with nearby wireless-enabled devices such as desktop computers, notebooks, handheld devices, and the like. Each access point has a certain range and a wireless-enabled device must be within that range for effective communications to occur. Through the access points, the wireless-enabled devices gain access to the network for data and file transfers, email, etc.
An exemplary configuration of a WLAN is shown in FIG. 1. As shown, configuration 10 includes four access points 12, 14, 16, and 18. Each access point is represented in FIG. 1 as a point, but is actually an electronic device as would be known by those of ordinary skill in the art. Each access point has a wireless coverage area. As shown, access point 12 has a coverage area 13 and, similarly, access points 14, 16 and 18 have coverage areas 15, 17 and 19, respectively. Preferably, adjacent coverage areas overlap geographically, at least slightly, to prevent dead or silent areas in which connectivity to the WLAN is lost.
The 802.11 standard noted above has spawned a number of task groups currently designated by the letters a through i. These task groups are involved with advancing the state of wireless technology in various ways. For example, the standard created by the b task group (referred to as the “802.11b standard”) specifies the use of the Industrial, Scientific, and Medical (ISM) frequency band. The ISM band is located between 2400 megahertz (MHz) and 2483.5 MHz. The 802.11b standard defines 14 channels designated as channels 1 through 14. Channel 1 is at 2412 MHz and the remaining channels 2-14 are spaced 5 MHz apart. Thus, channel 2 is at 2417 MHz, channel 3 is at 2422 MHz, and so on. Although 14 channels are defined in the 802.11b standard, the standard permits only channels 1 through 11 to be used in the U.S.
The radio in a wireless-enabled device automatically tunes its transceiver to the frequency of the access point having the strongest signal. To support roaming of wireless-enabled devices, the device's radio periodically scans the airwaves and reassociates with the access point having the strongest signal.
An important concept to note regarding channel assignments is that the channel actually represents the center frequency that the transceiver within the wireless-enabled device's radio and access point uses (e.g., 2412 MHz for channel 1 and 2417 MHz for channel 2). As stated above, there is only a five MHz separation between the center frequencies. It is generally regarded that an 802.11b signal occupies at most 22 MHz of the frequency spectrum with about one half of the bandwidth falling on each side of the center frequency. As a result, an 802.11b signal overlaps with several adjacent channel frequencies. This leaves only three channels (channels 1, 6, and 11 for the U.S.) that can be used simultaneously without causing interference between access points. When channel assignments are made to access points in a residence or business, care must be taken so as to ensure that only non-overlapping channels (1, 6, and 11 in the U.S.) are assigned to access points that are within range of each other to avoid contention between access points and wireless-enabled devices. Although the 802.11b standard does not specifically require the use of only three channels for access points, the ramification of the 802.11b teachings is that only three channels are used.
A single WLAN access point is capable of accommodating a limited number of simultaneous users. The number of users that can be accommodated in an area can be increased by increasing the number of access points in that area. However, as explained above, 802.11b-compliant access points within range of each other must be assigned one of only three non-overlapping channels (1, 6, and 11). As such, it is generally considered not possible to locate more than three access points in a small area having a high user density. A solution to this problem is needed that would permit a higher density of users to use a WLAN for a given frequency range. More specifically, it would be desirable to implement a WLAN which permits a higher density of users for the ISM band.